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Star Grounding Example


 
6/1/2000 2:21 AM
R.G.
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Star Grounding Example
With all the talk about star grounding, I finally got out my schemos from my Vox Pacemaker experiment and put them up at GEO with how I redid my amp. This is the first part of a more-definitive article on the rules for practical star grounding.  
 
It did run the hum down on the amp a lot, even if it was a lot of work. The worst problem is that the circuit grounds were just taken to the nearest grounded leg of a terminal strip. I actually had to splice in more terminal strips to have isolate ground collection points out in the amp.  
 
It's important to define the star ground point as a place that has no connection to the chassis. That way you can check your work with an ohmmeter to chassis, and when everything checks out, put that final wire in.  
 
The last wire might be a diode bridge paralleled by a 10 ohm resistor and a cap, too.  
 
I'm in the process of doing illustrations of rewiring some of the Fender chassis layouts for star ground. I'll post when I get that done.
 
6/1/2000 5:34 AM
Steve Ahola
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R.G.  
 
It's important to define the star ground point as a place that has no connection to the chassis. That way you can check your work with an ohmmeter to chassis, and when everything checks out, put that final wire in.  
 
 
    It is a good idea to do that test whenever you isolate jacks with those fiber washers... in a few cases I have had them slip around a bit so that the sleeve was touching the chassis even with the shoulder washers. I will check them out with a DMM before connecting them to my star ground point (which is unfortunately bolted down to the chassis with star washers... :( )  
 
--Thanks!  
 
Steve Ahola  
 
P.S. With cathode biased power tubes, should you run the cathode resistor and bypass cap to the star ground point, or is it okay to just bolt it to the chassis? I think that we had asked about the speaker jacks, too, and the bias circuit for a fixed bias amp... Someone had said that those connections could be omitted from the star ground scheme but I'd appreciate your input on this.
 
 
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6/1/2000 12:11 PM
Peter S
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Steve,  
One way to prevent the jacks from touching the sedes of the holes I use sometimes is to heat a thin strip od shrink wrap around the threads.  
PS
 
6/1/2000 1:32 PM
R.G.
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quote:
"With cathode biased power tubes, should you run the cathode resistor and bypass cap to the star ground point, or is it okay to just bolt it to the chassis?"
 
 
I personally would isolate it from chassis and run a wire to the star ground, as I'm sure that would work. However...  
 
This brings up the question of how much is enough. The point of star grounding is to keep signal circuits with NO common mode noise rejection (that is, all the common tube gain stages) from picking up IR noise from other currents running through a shared ground.  
 
Obviously, the most critical stage is an input stage, and the closer you go to the output stage, the bigger the signal level and the less the milli- and micro-volt signals in the shared ground will be.  
 
So you can achieve some reduction by breaking either end out of the shared-ground chassis. Break the small signal stages out of chassis ground and they don't pick up the IR noise from the high current stages that you've left in the chassis; Break the high current stages out and the currents remaining in the chassis get a lot smaller. The shared-resistance ground noise gets smaller either way. Is it enough to leave the output stage currents in the chassis? For all but the most critical situations, yeah, probably.  
 
The ultimate ground noise reduction is achieved by removing ALL the ground returns from the chassis and returning them to a star, but let's face it, there are other ways to cook a frog, as Fender shows us all the time. The star ground method is not the only way to achieve lower noise and crosstalk, it's just guaranteed to do it - that is, it's sufficient, but it may not be necessary, depending on the circumstances.  
 
It's entirely possible that moving the input stages and reverb returns off the chassis might quiet things down just fine, depending on where everything else is inside the amp.  
 
quote:
"I think that we had asked about the speaker jacks, too, and the bias circuit for a fixed bias amp "
 
Yeah, I tried the speaker jack connected/not connected to the chassis, and for the life of me could not hear any difference. I don't know whether this was because I already had everything else off the chassis or whether it truly didn't matter at all. I guess I would take a somewhat rough and ready approach - if it can be isolated and star grounded with a reasonable amount of wiring, I'd do that. If not, I'd leave it to the end and see if the results were acceptable from a listening test.  
 
If you notice, I did that with the speed control ground on the tremolo. It was in a position that was truly awkward to run a separate ground wire to, so I decided I was done and just listened to the result. It was OK. Not perfect star grounding, but it works fine, so I left it there.  
 
Bias circuits are things that I consider sacred. Make a mistake in a bias circuit and you get big problems. I'd recommend lavishing great care on a bias circuit, and so while it may be perfectly find grounded to the chassis, I'd star ground it on principle. It *is* an alternate input to your output stage, so if there's hum or RF on your chassis, it's conducted into the inputs of your output tubes. I'd star the bias - unless actual listening says it works fine without it.
 
6/1/2000 2:37 PM
Steve Slick
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All,  
 
Once again, R.G. gives us something that we can't get anywhere else. For free.  
 
Jeeze. I say we petition the Pope to make this guy a saint. What'd ya say?  
 
Thanks Again R.G.,  
 
Steve Slick
 
6/1/2000 2:46 PM
R.G.
quote:
"I say we petition the Pope to make this guy a saint. What'd ya say?"
 
 
...er... does that mean I'd have to be celibate???
 
6/1/2000 4:54 PM
anon
On the basis of historical examples, I'd  
say Not At All.
 
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